In the search for ways to better insulate windows, particularly of existing structures, it has been the practice to add new supplemental windows to the outside of the structure along with a new outer framing member surrounding the added window. The installation of such outside windows is of course difficult and laborious and the use of a new frame adds to its cost. Moreover such outside windows interfere with the use of the windows for ventilation purposes where the existing windows are opened by swinging outwardly. Suggestions have also been made for the addition of supplemental windows inside existing windows thereby providing more convenience of installation. However such supplemental windows are not adapted to provide a tight seal around a somewhat irregularly shaped window case, without the presence of an additional peripheral frame.